Bidirectional shift register and image display device using the same

ABSTRACT

A plurality of cascaded unit register circuits which comprises a bidirectional shift register include main stages and dummy stages at the top before the main stages and dummy stages at the bottom after the main stages. A k-th stage outputs a pulse P k  in synchronization with a clock signal with a reference point N 1  being at H level. The main stages include terminals NSF and NSB for setting N 1  to H to which P k−1  and P k+1  are input, respectively, and terminals NRB and NRF for setting N 1  to L level to which P k−2  and P k+2  are input, respectively. The order of generation of clock signals is reversed according to the direction of a shift, and whether a start trigger signal is applied to a top stage or a bottom stage is switched. Top dummy stages do not have NRB. Bottom dummy stages do not have NRF.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2010-142838 filed on Jun. 23, 2010, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bidirectional shift register whichcan switch the order of output of pulses and an image display devicewhich uses the bidirectional shift register to drive the respectivescanning lines.

2. Description of the Related Art

Higher resolution of a liquid crystal display device is materialized byimproving the arrangement density of pixels in a display portionthereof. As the arrangement density is improved, the arrangement pitchof various kinds of signal lines for supplying signals to pixel circuitsbecomes finer. Gate lines provided correspondingly to scanning lines ofpixels are connected to gate line driving circuits on sides of a displayregion. A gate line driving circuit includes a shift register foroutputting to the respective scanning lines in sequence a voltage whichenables writing data to a pixel circuit. As the resolution becomeshigher, unit register circuits included in the respective stages of theshift register are also required to be reduced in size.

Ordinarily, a voltage is applied to gate lines in a top to bottom orderof an image correspondingly to an order of input of image data in avertical scanning direction. If the shift register may be drivenbidirectionally, input image data may be written to pixel circuits in abottom to top order when the scan line goes from the bottom to the top.This enables change in the direction of a displayed image with amechanism that is simpler than that in a structure in which a framememory for buffering image data or the like is provided and the order ofthe image data is changed thereby.

A shift register used in a gate line driving circuit or the likeincludes cascaded unit register circuits in a plurality of stages.Basically, operation in which the respective unit register circuits inthe respective stages output a pulse once in an order from one end tothe other end of the row of the unit register circuits is performed, theoperation being linked with vertical scanning or the like.

FIG. 14 is a circuit diagram illustrating a basic structure of a unitregister circuit (see Japanese Patent Application Laid-open Nos.2004-157508 and 2009-272037). In a unit register circuit in an n-thstage, an output transistor M1 is connected between an output terminal(GOUT[n]) and a clock signal source CK, while a transistor M2 isconnected between the terminal (GOUT[n]) and a power supply VOFF. FIG.15 is a signal waveform chart illustrating operation of the unitregister circuit illustrated in FIG. 14. When an output pulse GOUT[n−1]in the previous stage is input to the unit register circuit, a node N3(one end of a capacitor C) connected to a gate of M1 is connected to apower supply VON, and a potential of the node N3 is pulled up to a High(H) level which is a potential at which the transistor is turned on.When the node N3 is at the H level, a node N4 is connected to the powersupply VOFF to be set to a Low (L) level which is a potential at whichthe transistor is turned off, thereby turning off the transistor M2. Inthis way, the unit register circuit is in a set state. In this state,when a clock signal CKV (CK) transitions from the L level to the Hlevel, the potential of the node N3 is further pulled up via thecapacitor C connected between the gate and a source of the outputtransistor M1, and the clock signal CKV at the H level is output fromthe terminal GOUT[n].

On the other hand, in the case of transition of the clock signal CKVfrom the H level to the L level, the potential of the node N3 is pulleddown and a voltage at the output terminal GOUT[n] is also pulled down.Here, linked with a rising edge of a clock signal CKB to an (n+1)thstage, a pulse is generated in an output signal GOUT [n+1] in thesubsequent stage, which is input to the unit register circuit in then-th stage. The pulse of GOUT[n+1] pulls down the potential of the nodeN3. This pulls up a potential of the node N4, the transistor M2 isturned on, and the output terminal is connected to the power supplyVOFF. By the operation, a pulse of the output signal GOUT[n] is output.

SUMMARY OF THE INVENTION

In order to materialize bidirectional drive, both a structure used whenthe drive is in a forward direction and a structure used when the driveis in a reverse direction are provided in the unit register circuit, anda switch element for switching the directions is built in the unitregister circuit. However, a bidirectional shift register which adoptssuch a unit register circuit has problems that miniaturization thereofis difficult and that it is not suitable for higher resolution of animage display device. On the other hand, a unit register circuit whichdoes not switch the structures according to the direction of the shiftmay be more liable to present a problem with regard to the stability ofoperation of the circuit as compared with a unit register which switchesthe structures.

Further, at the timing at which an output pulse ends in each of thestages, the potential of the node N3 is abruptly pulled down from apotential higher than the H level to the L level, and the transistor M2is turned on. Such operation is liable to cause unstable behavior atthat timing unless timings and waveforms of signals relating to theoperation are controlled accurately. For example, there is a problemthat the transistor M2 may begin to be turned on before the outputtransistor M1 is completely turned off, which causes a through currentto flow from the power supply VON to the power supply VOFF.

The present invention has been made to solve the above-mentionedproblems, and an object of the present invention is to provide abidirectional shift register which may perform stable bidirectionalshift operation with a simple circuit structure and an image displaydevice using the bidirectional shift register.

{Solution to Problem}

A bidirectional shift register according to one aspect of the presentinvention includes: a shift register portion including unit registercircuits each of which has an output terminal for outputting a pulse andwhich are cascaded in m stages, where m is an integer which is equal toor larger than 6, for outputting the pulse from a column of the outputterminals in a shift order which is one of a forward direction and areverse direction; a clock signal generating portion for supplyingn-phase clock pulses, where n is an integer which is equal to or largerthan 3, to the stages of the shift register portion in sequence in theforward direction in forward shift operation of the shift registerportion and in the reverse direction in backward shift operation; and atrigger signal generating portion for generating a forward directiontrigger signal at a start of forward shift and generating a reversedirection trigger signal at a start of backward shift. The cascaded unitregister circuits includes main stages that are unit register circuitsin which a load to be driven is connected to the output terminal anddummy stages that are unit register circuits in which a load to bedriven is not connected to the output terminal, the main stages forminga successive column and the dummy stages being connected at a top beforethe column of the main stages and at a bottom after the column of themain stages in the cascade. When αf, βb, βf, and βb are natural numberswhich satisfy both αf<βb<n and αb<βf<n, the unit register circuit in ak-th stage, where k is an integer which satisfies 1≦k≦m, includes: anoutput circuit for outputting an output pulse P_(k) in synchronizationwith the n-phase clock pulse which is input to the unit register circuitwith a reference point of the unit register circuit being at a firstpotential; both a forward direction set terminal to which an outputpulse P_(k−αf) is input as a set signal and a reverse direction setterminal to which an output pulse P_(k+αb) is input as a set signal; atleast one of a forward direction reset terminal to which an output pulseP_(k−βf) is input as a reset signal and a reverse direction resetterminal to which an output pulse P_(k−βb) is input as a reset signal; aset circuit for setting a potential of the reference point to the firstpotential when the set signal is input; and a reset circuit for settingthe potential of the reference point to a second potential when thereset signal is input. Each of the main stages includes both the forwarddirection reset terminal and the reverse direction reset terminal. Anumber of the top dummy stages provided is βb, each of the top dummystages includes only the forward direction reset terminal as the resetterminal, a number of the bottom dummy stages provided is βf, and eachof the bottom dummy stages includes only the reverse direction resetterminal as the reset terminal. The forward direction trigger signalinstead of the output pulse P_(k−αf) is input as the set signal to theforward direction set terminal in top of stages of the dummy stages, andthe reverse direction trigger signal instead of the output pulseP_(k+αb) is input as the set signal to the reverse direction setterminal in bottom αb stages of the dummy stages.

An image display device according to the present invention includes: aplurality of pixel circuits arranged in matrix correspondingly to aplurality of scanning lines; a plurality of gate signal lines providedfor the plurality of scanning lines, respectively, for supplying a gatesignal for controlling writing of image data to the plurality of pixelcircuits; and a gate signal line driving circuit including theabove-mentioned bidirectional shift register according to the presentinvention in which the plurality of gate signal lines are connected tothe output terminals in the main stages, respectively, for generatingthe gate signal based on the pulse which is output from the main stagesof the shift register portion.

According to the present invention, there may be obtained abidirectional shift register which may perform stable bidirectionalshift operation with a simple circuit structure and an image displaydevice using the bidirectional shift register.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram illustrating a structure of an imagedisplay device according to a first embodiment;

FIG. 2 is a schematic diagram illustrating a structure of abidirectional shift register according to the first embodiment;

FIG. 3 is a schematic circuit diagram of a unit register circuit in afirst stage of the bidirectional shift register according to the firstembodiment;

FIG. 4 is a schematic circuit diagram of a unit register circuit in asecond stage of the bidirectional shift register according to the firstembodiment;

FIG. 5 is a schematic circuit diagram of a unit register circuit in aλth stage of the bidirectional shift register according to the firstembodiment;

FIG. 6 is a schematic circuit diagram of a unit register circuit in an(N−1)th stage of the bidirectional shift register according to the firstembodiment;

FIG. 7 is a schematic circuit diagram of a unit register circuit in anN-th stage of the bidirectional shift register according to the firstembodiment;

FIG. 8 is a timing diagram illustrating waveforms of various signals inforward shift operation of the bidirectional shift register according tothe first embodiment;

FIG. 9 is a timing diagram illustrating waveforms of various signals inbackward shift operation of the bidirectional shift register accordingto the first embodiment;

FIG. 10 is a schematic diagram illustrating a structure of abidirectional shift register according to a second embodiment;

FIG. 11 is a schematic circuit diagram of a unit register circuit in afirst stage and a second stage of the bidirectional shift registeraccording to the second embodiment;

FIG. 12 is a schematic circuit diagram of a unit register circuit in aλth stage of the bidirectional shift register according to the secondembodiment;

FIG. 13 is a schematic circuit diagram of a unit register circuit in an(N−1)th stage and an N-th stage of the bidirectional shift registeraccording to the second embodiment;

FIG. 14 is a circuit diagram illustrating a structure of a conventionalunit register circuit; and

FIG. 15 is a signal waveform chart illustrating operation of theconventional unit register circuit.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention (hereinafter simplyreferred to as embodiments) are described with reference to the attacheddrawings.

First Embodiment

FIG. 1 is a schematic diagram illustrating a structure of an imagedisplay device 10 according to a first embodiment. The image displaydevice 10 is, for example, a liquid crystal display. The image displaydevice 10 includes a plurality of pixel circuits 12, gate line drivingcircuits 14, a data line driving circuit 16, and a control circuit 18.

The pixel circuits 12 are arranged in a display portion in matrix so asto correspond to pixels.

A plurality of gate signal lines 20 arranged in a vertical direction(column direction) are connected to the gate line driving circuits 14.Each of the gate signal lines 20 is connected to a plurality of pixelcircuits 12 arranged in a horizontal direction (row direction). The gateline driving circuits 14 output a gate signal to the plurality of gatesignal lines 20 in sequence to render pixel circuits 12 connected to thegate signal lines 20 data writable.

A plurality of data lines 22 arranged in the horizontal direction areconnected to the data line driving circuit 16. Each of the data lines 22is connected to a plurality of pixel circuits 12 arranged in thevertical direction. The data line driving circuit 16 generates, fromimage data corresponding to one scanning line, signals corresponding toa plurality of pixels forming the scanning line and outputs the signalsto the plurality of data lines 22. The pixel signals which are output tothe respective data lines 22 are written to pixel circuits 12 which arerendered writable by a gate signal, and the respective pixel circuits 12control the amount of light emitted from the pixels according to thewritten pixel signals.

The control circuit 18 controls operation of the gate line drivingcircuits 14 and of the data line driving circuit 16.

The image display device 10 includes as the gate line driving circuits14 a gate line driving circuit 14L provided on a left side of thedisplay portion and a gate line driving circuit 14R provided on a rightside of the display portion. The gate line driving circuit 14R suppliesa gate signal to odd-numbered gate signal lines 20 while the gate linedriving circuit 14L supplies a gate signal to even-numbered gate signallines 20. The gate line driving circuits 14 and the control circuit 18form the bidirectional shift register and the order of supplying a gatesignal to the gate signal lines 20 may be switched between a forwarddirection from a top to a bottom of the display portion and a reversedirection from the bottom to the top of the display portion.

FIG. 2 is a schematic diagram illustrating a structure of abidirectional shift register 30 used for scanning the gate signal lines20 of the image display device 10. The bidirectional shift register 30includes a shift register portion 32, a clock signal generating portion34, and a trigger signal generating portion 36. The shift registerportion 32 is provided in the gate line driving circuits 14 while theclock signal generating portion 34 and the trigger signal generatingportion 36 are provided in, for example, the control circuit 18. Theshift register portion 32 includes cascaded unit register circuits 38 ina plurality of stages.

FIG. 2 illustrates by way of example a portion of the bidirectionalshift register 30 which relates to the shift register portion 32provided in the gate line driving circuit 14R on the right side. Thegate line driving circuit 14R drives in sequence the odd-numbered gatesignal lines 20, that is, every other gate signal lines 20 at timingsshifted by 2H (H is a horizontal scanning period for one row). On theother hand, the gate line driving circuit 14L drives in sequence theeven-numbered gate signal lines 20 at timings shifted by 1H from theodd-numbered gate signal lines 20. The shift register portion 32 of agate line driving circuit 14 on one side is adapted to be driven byfour-phase clock signals. The drive on one side is shifted by 1H fromthe other side, and thus, the clock signal generating portion 34generates eight-phase clock signals V1-V8. In each of the clock signals,a pulse having a cycle of 8H and a width of 2H is generated, and clocksignals which are adjacent to each other in terms of phase, that is, Vjand V(j+1) are set to have a phase difference of 1H. More specifically,clock pulses which are adjacent to each other in terms of phase overlapby 1H of a 2H period during which the clock pulses are held at an Hlevel. The clock signal generating portion 34 has a first set of signalsV1, V3, V5, and V7 and a second set of signals V2, V4, V6, and V8. Inboth of the sets, the signals are shifted by 2H. The clock signalgenerating portion 34 supplies the first set to the gate line drivingcircuit 14R and supplies the second set to the gate line driving circuit14L. The unit register circuit 38 in each of the stages is correlatedwith one clock signal (output control clock signal) having a phase thatdefines the timing of an output pulse in the stage among the clocksignals of the plurality of phases.

In forward shift operation of the shift register portion 32, the clocksignal generating portion 34 generates clock pulses in sequence in theforward direction, that is, in the order of V1, V2, . . . , V8, V1, . .. . On the other hand, in backward shift operation, the clock signalgenerating portion 34 generates clock pulses in sequence in the reversedirection, that is, in the order of V8, V7, . . . , V1, V8, . . . . Theclock signal generating portion 34 supplies the generated clock pulsesto the respective stages of the shift register portions 32 of the gateline driving circuit 14R and the gate line driving circuit 14L,respectively. The clock signal generating portion 34 supplies clocksignals in the order of V1, V3, V5, V7, V1, . . . with different phasesfor the respective stages as the output control clock signals to thegate line driving circuit 14R from the top stage (upper side) down tothe bottom stage (lower side). With regard to the gate line drivingcircuit 14L, the order is set to be V2, V4, V6, V8, V2, . . . .

The trigger signal generating portion 36 generates a forward directiontrigger signal VSTF at the start of a forward shift and generates areverse direction trigger signal VSTB at the start of a backward shift.More specifically, the trigger signal generating portion 36 generates apulse which rises to the H level in the signal VSTF at the start of aforward shift and generates a pulse which rises to the H level in thesignal VSTB at the start of a backward shift.

As described above, the shift register portion 32 is constructed toinclude the plurality of cascaded unit register circuits 38. Each of theunit register circuits 38 outputs a pulse from an output terminalthereof. The shift register portion 32 outputs a pulse from the unitregister circuits 38 in the respective stages. In the forward shiftoperation, the shift register portion 32 outputs a pulse in sequencefrom the top stage of the unit register circuits 38, while, in thebackward shift operation, the shift register portion 32 outputs a pulsein sequence from the bottom stage of the unit register circuits 38.

The unit register circuits 38 in the plurality of stages which form theshift register portion 32 include main stages having output terminalsthat are connected to the gate signal lines 20, respectively, and dummystages which are added to the top and the bottom of the column of themain stages and which are not connected to the gate signal lines 20. Thetotal number of stages in the shift register portion 32 is denoted as N.The value N of the total number of stages is determined by the number ofthe scanning lines of the image display device 10, that is, the numberof the gate signal lines 20, and the number of the top dummy stages andthe bottom dummy stages. In this embodiment, two dummy stages areprovided at the top of the gate line driving circuit 14 and two dummystages are provided at the bottom of the gate line driving circuit 14.When an output of the unit register circuit 38 in a k-th stage on thegate line driving circuit 14R side is expressed as G(2k−1) (k is anatural number which satisfies 1≦k≦N), outputs G1, G3, G(2N−3), andG(2N−1) from the dummy stages are not output to the gate signal lines 20while an output G(2λ−1) from a λth stage (λ is a natural number whichsatisfies 3≦λ≦N−2) which is a main stage is output to a correspondinggate signal line 20.

It is to be noted that, when an output of the unit register circuit 38in the k-th stage on the gate line driving circuit 14L side is expressedas G(2k), outputs G2, G4, G(2N−2), and G(2N) from the dummy stages arenot output to the gate signal lines 20 while an output G(2λ) from theλth stage which is a main stage is output to a corresponding gate signalline 20.

FIG. 2 illustrates connections of input/output terminals of the unitregister circuits 38. It is to be noted that, for simple illustration,clock signals are denoted as, for example, V(2λ−1), where a clock signalVζ having a phase that is expressed by a number ζ exceeding 8 means aclock signal Vξ having a phase that is expressed by a remainder ξ leftwhen ζ is divided by 8.

FIGS. 3 to 7 are schematic circuit diagrams of the unit registercircuits 38. FIG. 3, FIG. 4, FIG. 5, FIG. 6, and FIG. 7 illustrate theunit register circuits 38 in a first stage, a second stage, the λthstage, an (N−1)th stage, and an N-th stage, respectively.

First, a structure of the unit register circuit 38 in the main stage(λth stage) illustrated in FIG. 5 is described, and then, a structure ofthe unit register circuits 38 in the dummy stages (k=1, 2, N−1, and N)is described mainly with reference to points different from thestructure in the main stage.

The unit register circuit 38 in the λth stage includes n-channeltransistors T1F, T1B, T2 to T6, T7F, T7B, T9F, T9B, T10F, and T10B andcapacitors C1 and C3.

The unit register circuit 38 in the λth stage has an output terminalNOUT(λ) for outputting a pulse G(2λ−1) of its own stage. The unitregister circuit 38 further has, as terminals to which a pulse inanother stage is input, a forward direction set terminal NSF(λ), areverse direction set terminal NSB(λ), a forward direction resetterminal NRF(λ), and a reverse direction reset terminal NRB(λ). Anoutput signal G(2λ−3) is input to the terminal NSF(λ) from the (λ−1)thstage, an output signal G(2λ+1) is input to the terminal NSB(λ) from the(λ+1)th stage, an output signal G(2λ+3) is input to the terminal NRF(λ)from the (λ+2)th stage, and an output signal G(2λ−5) is input to theterminal NRB(λ) from the (λ−2)th stage.

Further, output signals V(2λ−1) and V(2λ+3) are input to the unitregister circuit 38 in the λth stage from the clock signal generatingportion 34.

Further, to the unit register circuits 38, the forward direction triggersignal VSTF and the reverse direction trigger signal VSTB are input fromthe trigger signal generating portion 36, and a voltage at the H levelis supplied from a power supply VGH and a voltage at the L level issupplied from a power supply VGL.

A drain of the output transistor T5 is connected to a signal line of theoutput control clock signal V(2λ−1) and a source of the outputtransistor T5 is connected to the output terminal NOUT(λ), and theconduction of the transistor T5 is controlled according to the potentialof a reference point N1 connected to a gate of the transistor T5. Thecapacitor C1 is connected between the gate and the source of thetransistor T5. The transistor T5 and the capacitor C1 function as anoutput circuit which outputs the output pulse G(2λ−1) of its own stagein synchronization with the clock pulse V(2λ−1) with the node N1 as thereference point being at the H level.

Further, a drain of the transistor T6 is connected to the outputterminal NOUT(λ) and a source that is connected to the power supply VGL,and on/off of the transistor T6 is controlled according to the potentialof a node N2 connected to a gate of the transistor T6. The capacitor C3is connected between the node N2 and the power supply VGL.

The reference point N1 is connected to the terminals NSF(λ) and NSB(λ)via the diode-connected transistors T1F and T1B, respectively. Thetransistors T1F and T1B function as a set circuit which sets thereference point N1 to the H level when an output pulse of another stageis input to the terminal NSF(λ) or NSB(λ).

The transistors T2, T9F, and T9B which are connected between thereference point N1 and the power supply VGL so as to be in parallel toone another function as switch elements which providecontinuity/discontinuity between the reference point N1 and the powersupply VGL. A gate of the transistor T2 is connected to the node N2, agate of the transistor T9F is connected to the terminal NRF(λ), and agate of the transistor T9B is connected to the terminal NRB(λ). When thepotential of the node N2, the terminal NRF(λ), or the terminal NRB(λ) isat the H level, these transistors set the potential of the referencepoint N1 to the L level. In particular, the transistors T9F and T9Bfunction as a reset circuit which sets the reference point N1 to the Llevel when an output pulse of another stage is input to the terminalNRF(λ) or NRB(λ).

Here, the node N2 is set to the H level except for a period during whichthe reference point N1 is set to the H level. The transistor T2 is ONduring a period in which the node N2 is at the H level, and thus, thecontinuity is maintained for a relatively long time. As a result, athreshold voltage Vth of the transistor T2 is shifted to the positivedirection, and the ability of the transistor T2 to fix the referencepoint N1 to the L level is lowered. On the other hand, a pulse of theclock signal V(2λ−1) is applied to the drain of the transistor T5 evenoutside the set period of the reference point N1 (output period of theλth stage), and the pulse has a function to pull up the potential of thereference point N1 via a gate-drain capacitance Cgd of the transistorT5. In particular, as described below, at least the size of thetransistors T5 in the main stages is required to be large, and thus, thegate-drain capacitance Cgd becomes larger accordingly and the pulled-upamount of the potential of the reference point N1 becomes largeraccordingly. Therefore, the transistors T9F and T9B are provided so thatthe reference point N1 is preferably reset to the L level.

The transistors T3, T10F, and T10B which are connected between the nodeN2 and the power supply VGH so as to be in parallel to one anotherfunction as switch elements which provide continuity/discontinuitybetween the node N2 and the power supply VGH. A gate of the transistorT3 is connected to a signal line of a clock signal (2λ+3), a gate of thetransistor T10F is connected to a signal line of the forward directiontrigger signal VSTF, and a gate of the transistor T10B is connected to asignal line of the reverse direction trigger signal VSTB. When thepotential of the clock signal (2λ+3), the forward direction triggersignal VSTF, or the reverse direction trigger signal VSTB is at the Hlevel, these transistors set the potential of the node N2 to the Hlevel.

The transistors T4, T7F, and T7B which are connected between the node N2and the power supply VGL so as to be in parallel to one another functionas switch elements which provide continuity/discontinuity between thenode N2 and the power supply VGL. A gate of the transistor T4 isconnected to the node N1, a gate of the transistor T7F is connected tothe terminal NSF(λ), and a gate of the transistor T7B is connected tothe terminal NSB(λ). When the potential of the node N1, the terminalNSF(λ), or the terminal NSB(λ) is at the H level, these transistors setthe potential of the node N2 to the L level.

Next, the unit register circuits 38 in the dummy stages are described.The unit register circuits 38 in the first stage and in the second stageillustrated in FIG. 3 and FIG. 4, respectively, do not have thetransistor T9B, and, correspondingly to this, do not have the terminalNRB, which is described below. It is to be noted that these first andsecond stages have the terminals NSF, NSB, and NRF. Similarly to thecase of the λth stage, a corresponding output signal in another stage isinput to these terminals except the terminal NSF(1). On the other hand,no corresponding output signal in another stage exists with regard tothe terminal NSF(1). The terminal NSF is provided for inputting in theforward shift operation a signal which sets the reference point N1 tothe H level in preparation for generating an output pulse. To theterminal NSF(1), a pulse of the forward direction trigger signal VSTF isinput from the trigger signal generating portion 36 at the start of aforward shift.

The unit register circuit 38 in the first stage is different from theunit register circuit 38 illustrated in FIG. 5 also in not having thetransistor T10F.

The unit register circuits 38 in the (N−1)th stage and in the N-th stageillustrated in FIG. 6 and FIG. 7, respectively, do not have thetransistor T9F, and, correspondingly to this, do not have the terminalNRF, which is described below. It is to be noted that these (N−1)th andN-th stages have the terminals NSF, NSB, and NRB, and similarly to thecase of the λth stage, a corresponding output signal in another stage isinput to these terminals except the terminal NSB(N). On the other hand,no corresponding output signal in another stage exists with regard tothe terminal NSB(N). The terminal NSB is provided for inputting in thebackward shift operation a signal which sets the reference point N1 tothe H level in preparation for generating an output pulse. To theterminal NSB(N), a pulse of the reverse direction trigger signal VSTB isinput from the trigger signal generating portion 36 at the start of abackward shift.

The unit register circuit 38 in the N-th stage is different from theunit register circuit 38 illustrated in FIG. 5 also in not having thetransistor T10B.

The output terminal NOUT in a main stage is connected to the gate signalline 20 and to the plurality of pixel circuits 12 as loads to be driven.As the length of the gate signal line 20 increases due to a largerscreen and as the number of the pixel circuits 12 connected to the gatesignal line 20 increases due to higher resolution, the loads to bedriven become heavier. The output transistor T5 in a main stage isrequired to have a driving ability corresponding to the loads, and isdesigned to have, for example, a large gate width (channel width). Forexample, the output transistor T5 in a main stage is designed to have achannel width as large as about 5,000 μm. On the other hand, dummystages are not connected to the gate signal lines 20. Thus, the drivingability of the output transistor T5 in a dummy stage is set to be lowerthan that in a main stage. For example, the output transistor T5 in adummy stage is set to have a channel width of about 500 μm, which is1/10 of that of the output transistor T5 in a main stage. In this way,the size of the transistors T5 in the dummy stages reduces, and thus,the unit register circuits 38 in the dummy stages may be miniaturized.Further, power consumption of the dummy stages may be reduced.

In the above, the structure of the gate line driving circuits 14 isdescribed taking as an example the gate line driving circuit 14R on theright side for driving the odd-numbered gate signal lines 20. Astructure of the gate line driving circuit 14L on the left side fordriving the even-numbered gate signal lines 20 is similar to that of thegate line driving circuit 14R on the right side, but is describedbriefly for confirmation. For example, the λth stage of the shiftregister portion 32 is connected to the gate signal line 20 in the(2λ−1)th row in the gate line driving circuit 14R on the right side,while the λth stage of the shift register portion 32 is connected to thegate signal line 20 in the (2λ)th row in the gate line driving circuit14L on the left side. In a forward shift, the gate signal line 20 in the(2λ)th row is driven with the delay of 1H from the drive of the gatesignal line 20 in the (2λ−1)th row. As is easily conceived from thisrelationship, the output terminal NOUT(k) of the unit register circuit38 in the k-th stage (1≦k≦N) of the gate line driving circuit 14Loutputs a signal G(2k), and signals G(2λ−2), G(2λ+2), G(2λ+4), andG(2λ−4) are input to the terminals NSF(λ), NSB(λ), NRF(λ), and NRB(λ),respectively, in the main stages. Further, a signal V(2k) as the outputcontrol clock signal is input to the transistor T5, and a clock signalV(2k+4) is applied to the gate of the transistor T3.

Next, operation of the bidirectional shift register 30 is described.FIG. 8 is a timing diagram illustrating waveforms of various signals inthe forward shift operation.

The forward shift starts by, at the head of image signals for one frame,generation of pulses of forward direction trigger signals by the triggersignal generating portion 36 (at times t0 and t1). The trigger signalgenerating portion 36 generates, after generating a pulse of the forwarddirection trigger signal VSTF for driving the odd-numbered lines at thetime t0, a pulse of a forward direction trigger signal VSTF2 for drivingthe even-numbered lines at the time t1, which is delayed by 1H from t0.On the other hand, the reverse direction trigger signal VSTB for drivingthe odd-numbered lines and a reverse direction trigger signal VSTB2 fordriving the even-numbered lines are fixed at the L level.

In the unit register circuits 38 in the second to N-th stages, when apulse of the signal VSTF is input, the transistor T10F is turned on, thenode N2 is pulled up to the H level, and, as a result, the transistor T2is turned on to reset the reference point N1 to the L level. On theother hand, in the unit register circuits 38 in the first to (N−αb)thstages, when a pulse of the signal VSTB is input, the transistor T10B isturned on to reset the reference point N1 to the L level.

As described above, the clock signal generating portion 34 generates, inthe forward shift operation, the pulses in sequence in the forwarddirection. More specifically, the clock signal generating portion 34raises a pulse of a clock signal V(j+1) with a delay of 1H from a risingedge of a pulse of a clock signal Vj (j is a natural number whichsatisfies 1≦j≦7), and further, raises a pulse of the clock signal V1with a delay of 1H from a rising edge of a pulse of the clock signal V8.

Here, first, forward shift operation of the unit register circuit 38 ina main stage (λth stage) in the gate line driving circuit 14R isdescribed.

Before the operation in the λth stage, the first to (λ−1)th stages areoperated in sequence to output pulses having a width of 2H with a phasedifference of 2H therebetween. When a pulse of the output signal G(2λ−3)in the (λ−1)th stage is input to the terminal NSF(λ) (time t2), thereference point N1 is set to a potential (VGH-Vth (T1F)) correspondingto the H level, the transistor T5 is turned on, and the interterminalvoltage of the capacitor C1 is set to the same potential. Here, thetransistor T4 is turned on to set the node N2 to the L level. Further,here, the transistor T7F is also turned on, and thus, the node N2 is setto the L level faster than in a case in which only the transistor T4 isturned on. The potential of the node N2 is held in the capacitor C3. Thenode N2 is at the L level, and thus, the transistors T2 and T6 are in anoff state.

An output pulse in the (λ−1)th stage is generated in synchronizationwith a pulse of a clock V(2λ−3) (in FIG. 8, a pulse of the clock V7),and thus, at a time t3 which is delayed by 2H from the time t2, a pulseof a clock signal V(2λ−1) (in FIG. 8, a pulse of the clock signal V1) isinput to the λth stage. The pulse of the clock signal V(2λ−1) pulls upthe potential of the source of the transistor T5. Then, the potential ofthe node N1 is further pulled up by the bootstrap effect, and the pulseof the clock signal V(2λ−1) becomes a pulse of the signal G(2λ−1)without lowering the potential thereof to be output from the terminalNOUT(λ). The pulse of the signal G(2λ−1) is input to the terminal NSF inthe (λ+1)th stage, and sets the node N1 in that stage to the H level.

When, at a time t4, the pulse of the clock signal V(2λ−1) falls, thepulse of the signal G(2λ−1) also falls. On the other hand, the potentialof the reference point N1 is held at the H level.

At the time t4, the (λ+1)th stage outputs a pulse of the signal G(2λ+1)in synchronization with a pulse of a clock signal V(2λ+1). In this way,each of the stages outputs a pulse of its own with a delay of 2H fromthe output of a pulse in the preceding stage. The (λ+2)th stage whichreceives the output of the pulse in the (λ+1)th stage outputs a pulse ofthe signal G(2λ+3) at a time t5 which is delayed by 2H from the time t4.

In the λth stage, when, at the time t5, the pulse of the signal G(2λ+3)is input to the terminal NRF, the transistor T9F is turned on to resetthe reference point N1 to the L level. At the same time, a clock signalV(2λ+3) turns on the transistor T3 to pull up the node N2 to the Hlevel. As a result, the transistor T6 is turned on to connect the outputterminal NOUT(λ) to the power supply VGL.

It is to be noted that the transistor T3 is periodically turned on bythe clock signal V(2λ+3) also at times other than the time t5, andsatisfactorily holds the node N2 at the H level except for a period inwhich the reference point N1 is in a set state. This causes the outputterminal NOUT(λ) to be held at the L level except for the period inwhich the reference point N1 is set to the H level.

By the above-mentioned operation, during the period of 2H which precedesthe time t2, a pulse is input from the (λ−2)th stage to the terminalNRB(λ) to turn on the transistor T9B. The period is before the referencepoint N1 is set to the H level by the input of a pulse from the (λ−1)thstage to the terminal NSF(λ), and thus, the above-mentioned operation isnot affected. Further, during the period of 2H between the times t4 andt5, a pulse is input from the (λ+1)th stage to the terminal NSB(λ), anda potential at the H level is applied from the terminal NSB(λ) via thetransistor T1B to the reference point N1. The period is before thereference point N1 is reset to the L level by the input of a pulse fromthe (λ+2)th stage to the terminal NRF(λ), and thus, the above-mentionedoperation is not affected.

Further, the timing of setting the reference point N1 to the H level isafter a pulse which precedes a pulse at the time t3 by a cycle among theplurality of pulses of the clock signal V(2λ−1), and the timing ofresetting the reference point N1 to the L level is before a pulse whichis generated after a cycle, and therefore, a pulse is output from theterminal NOUT(λ) only once, which is in synchronization with the pulseat the time t3 among the pulses of the clock signal V(2λ−1).

As described above, a main stage receives an output pulse from theprevious stage to cause the reference point N1 to be in the set state,and receives an output pulse from the stage next to the subsequent stageto cause the reference point N1 to be in the reset state. However, thedummy stage as the first stage does not have the previous stage.Therefore, as described above, the first stage has a structure in whicha pulse of the forward direction trigger signal VSTF is input to theterminal NSF. The first stage receives a pulse of the signal VSTF whichis generated at the time t0 to set the reference point N1 to the Hlevel. Subsequent operation in the first stage is similar to that in theλth stage described above. The (N−1)th and N-th stages do not have thestage next to the subsequent stage. Further, in the first place, unlikethe main stages, the (N−1)th and N-th stages do not have the forwarddirection reset terminal NRF. Therefore, the reference point N1 in the(N−1)th and N-th stages cannot be reset similarly to that in the mainstages. In this embodiment, the reference point N1 in the (N−1)th andN-th stages is set to the H level at the end of the forward shiftoperation for one frame, and then is reset to the L level when thetransistors T10F and T2 are turned on in response to a pulse of thesignal VSTF which is generated at the start of the subsequent frame. Itis to be noted that this point is described in detail below.

The forward shift operation of the stages in the gate line drivingcircuit 14R is described above. The forward shift operation of thestages in the gate line driving circuit 14L is similar to that of thecorresponding stages in the gate line driving circuit 14R. However, itis to be noted that the stages in the gate line driving circuit 14Loperate with a delay of 1H from the corresponding stages in the gateline driving circuit 14R, respectively.

FIG. 9 is a timing diagram illustrating waveforms of various signals inthe backward shift operation.

The backward shift starts by, at the head of image signals for oneframe, generation of pulses of reverse direction trigger signals by thetrigger signal generating portion 36 (at times t0 and t1). The triggersignal generating portion 36 generates, after generating a pulse of thereverse direction trigger signal VSTB2 for driving the even-numberedlines at the time t0, the pulse of a reverse direction trigger signalVSTB for driving the odd-numbered lines at the time t1, which is delayedby 1H from t0. On the other hand, the forward direction trigger signalVSTF for driving the odd-numbered lines and the forward directiontrigger signal VSTF2 for driving the even-numbered lines are fixed atthe L level.

As described above, the clock signal generating portion 34 generates, inthe backward shift operation, the pulses in sequence in the reversedirection. More specifically, the clock signal generating portion 34raises the pulse of the clock signal Vj with a delay of 1H from a risingedge of the pulse of the clock signal V(j+1), and further, raises apulse of the clock signal V8 with a delay of 1H from a rising edge ofthe pulse of the clock signal V1.

The unit register circuit 38 in each of the stages of the shift registerportion 32 has a circuit structure in which a portion related to theterminal NSF and a portion related to the terminal NSB are symmetricalwith each other and in which a portion related to the terminal NRF and aportion related to the terminal NRB are symmetrical with each other.More specifically, according to the four-phase clocks used for drivingthe gate line driving circuit 14 on one side, in both the forward shiftoperation and the backward shift operation, the unit register circuit 38in each of the stages is adapted to receive at the terminal NSB anoutput pulse which is generated with an advance of one phase of theclock, that is, 2H, from that of its own stage to cause the referencepoint N1 to be in the set state, and is adapted to receive at theterminal NRB an output pulse which is generated with a delay of twophases of the clock, that is, 4H, from that of its own stage to causethe reference point N1 to be in the reset state. Further, both ends ofthe shift register portion 32, that is, the top dummy stages and thebottom dummy stages are in a symmetrical relationship in structure withrespect to shifts in opposite directions. More specifically, the topdummy stages in the backward shift operation function similarly to thebottom dummy stages in the forward shift operation, while the bottomdummy stages in the backward shift operation function similarly to thetop dummy stages in the forward shift operation. Therefore, if thecontrol circuit 18 switches the trigger signals and switches the orderof generation of the clock pulses, the shift register portion 32performs the backward shift operation similarly to the forward shiftoperation.

For example, in the N-th stage in the gate line driving circuit 14R, apulse of the reverse direction trigger signal VSTB is input to theterminal NSB at the time t1 and the reference point N1 is set to the Hlevel. After that, in synchronization with a pulse of a clock signalV(2N−1) which is generated first, a pulse is generated in an outputsignal G(2N−1). In this way, pulses are output from the stages insequence in the opposite order to that in the forward shift operation.

The backward shift operation is described in the above taking as anexample the gate line driving circuit 14R. The backward shift operationof each of the stages in the gate line driving circuit 14L is similar tothat of the corresponding stage in the gate line driving circuit 14R.However, each of the stages in the gate line driving circuit 14Lperforms operation with an advance of 1H from the corresponding stage inthe gate line driving circuit 14R.

Here, with its own stage being a starting point, another stage whichinputs a pulse to the reset terminal NRF is set to be a stage which isfarther than still another stage that inputs a pulse to the set terminalNSB, and, another stage which inputs a pulse to the reset terminal NRBis set to be a stage which is farther than still another stage thatinputs a pulse to the set terminal NSF. In this structure, in theforward shift operation, the pulses which are input to the terminals NSBand NRB that are related to the backward shift operation do not affectthe forward shift operation, and, similarly, in the backward shiftoperation, the pulses which are input to the terminals NSF and NRF thatare related to the forward shift operation do not affect the backwardshift operation. Therefore, for example, it is not necessary to providea switch for selectively accepting the inputs to the terminals NSF andNRF in the forward shift operation and selectively accepting the inputsto the terminals NSB and NRB in the backward shift operation. Morespecifically, the shift register portion 32 and the unit registercircuits 38 therein may have a basic circuit structure which is notswitched between one for the forward shift and one for the backwardshift. A transistor used as the switch is not necessary, and thus, thecircuit structure of the unit register circuit 38 becomes simpleraccordingly, which makes it easier to miniaturize the unit registercircuit 38. Further, a signal line for supplying a switching signal tothe transistor concerned in each of the stages is not required to bearranged along the shift register portion 32, and thus, the sizeincrease in a horizontal direction of the gate line driving circuits 14may be suppressed.

It is to be noted that, as described in the description of the forwardshift operation, in synchronization with the operation of resetting thereference point N1, a clock signal is used to turn on the transistor T3and the node N2 is pulled up to the H level. In this embodiment, clocksfor driving a gate line driving circuit 14 on one side are four-phaseclocks, and, for example, in the gate line driving circuit 14R, thereference point N1 is reset to a timing which is delayed by two phasesof the clock from the output control clock signal V(2k−1) to the outputtransistor T5 in its own stage. The clock signal which turns on thetransistor T3 at the timing of resetting the reference point N1 isV(2k+3) in the forward shift and V(2k−5) in the backward shift, whichare a common clock signal. This means that, in this embodiment, a clocksignal for controlling the transistor T3 is also not required to beswitched between one for the forward shift and one for the backwardshift.

In the above-mentioned embodiment, the top dummy stages do not have thereverse direction reset terminal NRB and the transistor T9B, and thebottom dummy stages do not have the forward direction reset terminal NRFand the transistor T9F. This structure is described. The reset terminalsNRF and NRB are used for inputting a signal which resets the referencepoint N1 to the L level after an output pulse is generated. By resettingthe reference point N1 to the L level, an output pulse is prevented frombeing generated by a pulse of an output control clock signal which isinput thereafter. Here, outputs of the first, second, (N−1)th, and N-thstages, which are dummy stages, are not used for driving the gate signallines 20. Further, outputs of the (N−1)th and N-th stages, which aredummy stages that operate after output pulses in the main stages aregenerated in a forward shift, and outputs of the first and secondstages, which are dummy stages that operate after output pulses in themain stages are generated in a backward shift, are not used as signalsfor setting the reference point N1 in another stage. Therefore, thesedummy stages which operate at the end of each shift operation do notpresent any specific problem even if the dummy stages repeatedlygenerate the output pulses according to repetition of clock pulses.Therefore, it is sufficient that the reference point N1 in the dummystages is caused to be in the reset state before the start of shiftoperation with regard to the subsequent frame. For this reason, in thisembodiment, as described in the description of the forward shiftoperation, the reference point N1 in the (N−1)th and N-th stages isreset by turning on the transistors T10F and T2 in response to a pulseof the signal VSTF which is generated at the start of the subsequentframe. Further, similarly, in the backward shift operation, thereference point N1 in the first and second stages is reset by turning onthe transistors T10B and T2 in response to a pulse of the signal VSTBwhich is generated at the start of the subsequent frame.

It is to be noted that, as described above, the transistors T9F and T9Bare primarily provided for the purpose of compensating for the loweredresetting ability of the transistor T2. However, in the dummy stages,the size of the transistor T5 is smaller than that in the main stagesand the capacitance Cgd is also smaller than that in the main stages,and thus, the pulled-up amount of the potential of the reference pointN1 described above is smaller than that in the main stages. Therefore,in the dummy stages, the reset may be performed onlyby the transistor T2without the assistance of the transistors T9F and T9B.

Now, consider a structure in which the reverse direction reset terminalNRB and the transistor T9B are added to the unit register circuit 38 inthe top dummy stages as means for resetting the reference point N1 inthe backward shift operation and the forward direction reset terminalNRF and the transistor T9F are added to the unit register circuit 38 inthe bottom dummy stages as means for resetting the reference point N1 inthe forward shift operation. In this case, it is not preferred toadditionally generate signals to be applied to the added reset terminalsNRF and NFB because, for example, the size of the control circuit 18 andthe like increases. In this regard, the circuit size does not increasein the structure in which the reverse direction trigger signal VSTB isinput to the terminal NRB added to the top dummy stages and the forwarddirection trigger signal VSTF is input to the terminal NRF added to thebottom dummy stages. In this structure, in the forward shift operation,the potential of the reference point N1 in the bottom dummy stages isreset by a pulse of the signal VSTF simultaneously with the start of thesubsequent frame, and, in the backward shift operation, the potential ofthe reference point N1 in the top dummy stages is reset by a pulse ofthe signal VSTB simultaneously with the start of the subsequent frame,to thereby materialize operation of a bidirectional shift register.

However, this structure may cause the following problem. In thisstructure, the reverse direction trigger signal VSTB is fixed to the Llevel in the forward shift operation, and the forward direction triggersignal VSTF is fixed to the L level in the backward shift operation, andthus, the transistor T9B in the first and second stages is held in theoff state in the forward shift operation and the transistor T9F in the(N−1)th and N-th stages is held in the off state in the backward shiftoperation. Such a transistor in which the drain and the source areapplied with a voltage and which is held in the off state for a longtime may have a change in transistor characteristics called Vth shift.More specifically, in an n-channel transistor, the threshold voltage Vthis liable to be lowered and a leakage current is liable to occur. TheVth shift is particularly liable to occur in an a-Si thin filmtransistor (TFT). When the transistor T9F or T9B causes a leakagecurrent, the potential of the reference point N1 is lowered in an outputperiod in which the reference point N1 should be at the H level, andhence output pulses are not generated. More specifically, in the forwardshift operation, the transistor T9B in the top dummy stages may cause aleakage current and output pulses may not be generated, and, in thebackward shift operation, the transistor T9F in the bottom dummy stagesmay cause a leakage current and output pulses may not be generated.Thus, an inconvenience that the shift operation cannot be continued mayoccur. Taking this into consideration, in the present invention, thereverse direction reset terminal NRB and the transistor T9B are notprovided in the top dummy stages and the forward direction resetterminal NRF and the transistor T9F are not provided in the bottom dummystages.

By the way, in the embodiment described above, a gate line drivingcircuit 14 on one side is driven by four-phase clock signals, and,basically, outputs of the (k−2)th stage, the (k−1)th stage, the (k+1)thstage, and the (k+2)th stage are input to the unit register circuit 38in the k-th stage so that the reference point N1 is set to the H levelby output pulses in (k−1)th stage and the (k+1)th stage and that thereference point N1 is reset to the L level by output pulses in the(k−2)th stage and the (k+2)th stage. Such a structure materializes abidirectional shift register which does not basically require switchbetween a circuit structure for the forward shift and a circuitstructure for the backward shift. Further, in such a structure, after anoutput pulse of each of the stages falls, the reference point N1 isreset from the H level to the L level. More specifically, after anoutput pulse of each of the stages ends, a subsequent set period inwhich the reference point N1 in the stage is held in the set state isprovided. By the existence of the subsequent set period, operation ofthe bidirectional shift register according to the present invention isnot operation in which the potential of the reference point N1 isabruptly pulled down from a potential higher than the H level to the Llevel and the transistor M6 is turned on, and thus, unstable operationdue to timing misalignment and deformation of the waveform of each ofthe signals such as a through current is less liable to occur.

Here, the present invention is not limited to the structure of theabove-mentioned embodiment. A generalized structure of the bidirectionalshift register according to the present invention is as follows. It isassumed that the shift register portion 32 is driven by n-phase clocksignals (n is an integer which is equal to or larger than 3), and αf,αb, βf, and βb are natural numbers which satisfy both αf<βb<n andαb<βf<n. Then, outputs of a (k−βb)th stage, a (k−αf)th stage, a (k+αb)thstage, and a (k+βf)th stage are input to the unit register circuit 38 inthe k-th stage so that the reference point N1 is set to the H level byoutput pulses in the (k−αf)th stage and the (k+αb)th stage and that thereference point N1 is reset to the L level by output pulses in the(k−βb)th stage and the (k+βf)th stage. Such a structure alsomaterializes a bidirectional shift register which does not basicallyrequire switch between circuit structures with the improved stability ofoperation.

It is to be noted that, because αf<βb and αb<βf, it follows that both βfand βb are equal to or larger than 2, and further, at least two mainstages are necessary for the forward shift and the backward shift. Itfollows that the total number N of stages in the bidirectional shiftregister is equal to or larger than 6.

In the top βb stages among the N stages, a reset signal in the backwardshift operation cannot be obtained from an output in another stage, and,in the bottom βf stages, a reset signal in the forward shift operationcannot be obtained from an output in another stage. Therefore, these topβb stages and bottom βf stages are set to be dummy stages. The top dummystages do not include the reverse direction reset terminal NRB and thereset circuit controlled by the terminal NRB (the transistor T9B in theabove-mentioned embodiment), and the bottom dummy stages do not includethe forward direction reset terminal NRF and the reset circuitcontrolled by the terminal NRF (the transistor T9F in theabove-mentioned embodiment).

In some cases, similarly to the case of the dummy stages in theabove-mentioned embodiment, a signal as an alternative to an outputpulse in another stage is also input to the terminal NSF, NSB, NRF, orNRB of the unit register circuits 38 in the dummy stages of thegeneralized shift register portion 32. More specifically, with regard toa bidirectional shift register having N stages, in the unit registercircuits 38 in the first to αf-th stages, the forward direction triggersignal is input to the terminal NSF, and the signal sets the referencepoint N1 to the H level at the start of the forward shift. Further, inthe unit register circuits 38 in the (N−αf+1)th to N-th stages, thereverse direction trigger signal is input to the terminal NSB, and thesignal causes the reference point N1 to be in the set state at the startof the backward shift.

The basic structure of the unit register circuit 38 may include thetransistors T10F and T2 as a circuit for setting the reference point N1to the L level as an initial setting by a pulse of the forward directiontrigger signal VSTF. However, as described above, in the first to αf-thstages, a pulse of the signal VSTF is used for setting the referencepoint N1 to the H level. Therefore, in the unit register circuits 38 inthe first to βf-th stages, the circuit structure does not have thetransistor T10F similarly to the case of the first stage in theabove-mentioned embodiment (see FIG. 3) so that the reference point N1is not reset to the L level. Similarly, the basic structure of the unitregister circuit 38 may include the transistors T10B and T2 as a circuitfor setting the reference point N1 to the L level as the initial settingby a pulse of the reverse direction trigger signal VSTB. However, asdescribed above, in the (N−αb+1)th to N-th stages, a pulse of the signalVSTB is used for setting N1 to the H level. Therefore, in the unitregister circuits 38 in the (N−βb+1)th to N-th stages, the circuitstructure does not have the transistor T10B similarly to the case of theN-th stage in the above-mentioned embodiment (see FIG. 7) so that thereference point N1 is not reset to the L level.

αf corresponds to a period from when the reference point N1 is set towhen an output pulse rises (preceding set period) in the forward shiftoperation while αb corresponds to the preceding set period in thebackward shift operation. When the preceding set period becomes long,the potential of the reference point N1 held by the capacitor C1 may belowered by a leakage current of the transistor T9F or T9B or the like,which may result in an inconvenience that, when a clock pulse is inputto the drain of the transistor T5, the potential of the gate of thetransistor T5 is not high enough to output a pulse from the terminalNOUT. Therefore, when there is concern about the above-mentionedinconvenience, for example, when the capacitance of the capacitor C1 isnot so large, it is preferred that, as in the above-mentionedembodiment, αf and αb be set to 1 to make short the preceding setperiod.

Further, from the viewpoint of symmetry between operation of the imagedisplay device 10 in the forward shift operation and that in thebackward shift operation, it is preferred that αf=αb and βf=βb besatisfied.

In the above-mentioned embodiment where n=4 and βf=βb=2, as describedabove, a clock signal used for controlling the transistor T3 may becommon between the forward shift operation and the backward shiftoperation. Such a structure in which a clock signal for controlling thetransistor T3 is common to the two directions is materialized whenβf+βb=n.

The unit register circuits are not limited to the ones illustrated inFIGS. 3 to 7. More specifically, in the main stages, the unit registercircuits may have another circuit structure which includes the forwarddirection set terminal NSF and the reverse direction set terminal NSB,the forward direction reset terminal NRF and the reverse direction resetterminal NRB, a set circuit for setting the potential of a referencepoint to a first potential when a set signal is input to the terminalNSF or the terminal NSB, a reset circuit for setting the potential ofthe reference point to a second potential when a reset signal is inputto the terminal NRF or the terminal NRB, and an output circuit foroutputting a pulse to an output signal in synchronization with a clockpulse which is input to the unit register circuit with the referencepoint being at the first potential. Further, the top dummy stages mayhave another circuit structure which includes, among the components inthe above-mentioned structure in the main stages, the components exceptthe reverse direction reset terminal NRB, and the bottom dummy stagesmay have another circuit structure which includes, among the componentsin the structure in the main stages, the components except the forwarddirection reset terminal NRF.

For example, a unit register circuit 60 in a second embodiment describedbelow is an example of such a circuit structure. Further, when theabove-mentioned condition for controlling the transistor T3 by a clocksignal common between the forward shift and the backward shift (βf+βb=n)is not satisfied, a circuit structure may be adopted in which a controlsignal to be applied to the gate of the transistor T3 is switchedbetween one for the forward shift and one for the backward shift, andthis is one modification of the unit register circuit. Further, in theabove-mentioned structure, the transistors T9F and T9B, which areswitches that provide continuity/discontinuity between the referencepoint N1 and the power supply VGL and function as a reset circuit forsetting the reference point N1 to the L level, are provided in theirsimplest form in which the drains thereof are directly connected to thereference point N1 and the sources thereof are directly connected to thepower supply VGL. However, the transistors T9F and T9B which providecontinuity/discontinuity between the reference point N1 and the powersupply VGL may be a part of the switch circuit including a plurality ofelements. In this structure, also, the transistor T9B in the top dummystages and the transistor T9F in the bottom dummy stages may beeliminated to prevent a Vth shift. Further, a problem which arises dueto the difference between a reset operation in the dummy stages in whicha pulse is output at the end of the shift operation and a resetoperation in the main stages is not limited to the one described above.In a bidirectional shift register in which the reset circuit has aproblem other than the above-mentioned one, such a problem may also beavoided by applying the structure according to the present invention.

It is to be noted that, in the above-mentioned embodiment, a case isdescribed in which a pulse of a clock signal which drives a gate linedriving circuit 14 on one side does not overlap another pulse in anadjacent phase. However, the present invention may also be applied to acase in which a pulse of a clock signal overlaps another pulse in anadjacent phase. In that case, it is necessary that a set signal to theterminal NSF or NSB and a reset signal to the terminal NRF or NRB do notoverlap when being input. More specifically, it is necessary that anoutput pulse in the (k−βb)th stage and an output pulse in the (k−αf)thstage do not overlap each other and an output pulse in the (k+αb)thstage and an output pulse in the (k+βf)th stage do not overlap eachother. By this, the following condition of αf, αb, βf, and βb isobtained:

αf+η≦βb<n and αb+κ≦βf<n,

where κ is the width of a clock pulse (κ≧1) when, for example, the phasedifference between the clock signals Vj and V(j+1) is 1.

A case in which an n-channel transistor is used as a transistor thatforms the bidirectional shift register 30 according to the presentinvention is described above, but the transistor may be a p-channel one.Further, the transistor may be TFT or may be MOSFET. A semiconductorlayer which forms the transistor basically may be any one ofmonocrystalline silicon, amorphous silicon (a-Si), and polycrystallinesilicon (poly-Si), and may also be an oxide semiconductor such as indiumgallium zinc oxide (IGZO).

Further, in the embodiment described above, the odd-numbered gate signallines 20 are connected to the gate line driving circuit 14R, theeven-numbered gate signal lines 20 are connected to the gate linedriving circuit 14L, and their respective bidirectional shift registers30 are driven each in a cycle of 2H with a phase difference of 1Htherebetween. On the other hand, the present invention may also beapplied to a structure in which the odd-numbered and even-numbered gatesignal lines 20 are connected to the gate line driving circuits 14 andone bidirectional shift register 30 outputs pulses in sequence to theodd-numbered and even-numbered gate signal lines 20 in a cycle of 1H.

Second Embodiment

In the following, like reference numerals are used to denote memberssimilar to those in the above-mentioned first embodiment and descriptionthereof is omitted for the sake of simplicity of description.

A schematic structure of an image display device 10 according to asecond embodiment is similar to that of the first embodiment describedwith reference to FIG. 1. The gate line driving circuit 14R drivesodd-numbered lines while the gate line driving circuit 14L driveseven-numbered lines.

FIG. 10 illustrates by way of example a structure of a portion whichrelates to the shift register portion 32 provided in the gate linedriving circuit 14R on the right side. N unit register circuits 60 arecascaded in the shift register portion 32. The number of the main stagesis N−4 and two dummy stages are provided at the top before the mainstages and two dummy stages are provided at the bottom after the mainstages. As described below, the unit register circuit 60 in thisembodiment has a circuit structure which is different from that of theunit register circuit 38 in the first embodiment in that a clock signaland a control signal to be input to each of the stages are differentfrom those in the first embodiment illustrated in FIG. 2. However,outputs of the stages and the way of cascading the stages are basicallysimilar to those in the first embodiment.

The clock signal generating portion 34 is similar to that in the firstembodiment. The eight-phase clock signals V1 to V8 are divided into twosets of four-phase clock signals. Among them, the group of V1, V3, V5,and V7 is supplied to the gate line driving circuit 14R. All of V1, V3,V5, and V7 are input to each of the unit register circuits 60. One clocksignal among them which is used as the output control clock signal isdetermined according to the location of the unit register circuit 60 inthe shift register portion 32.

FIGS. 11 and 13 are schematic circuit diagrams of the unit registercircuit 60. FIG. 11 illustrates the unit register circuit 60 in a topdummy stage, FIG. 12 illustrates the unit register circuit 60 in a mainstage (λth stage), and FIG. 13 illustrates the unit register circuit 60in a bottom dummy stage.

First, a structure of the unit register circuit 60 in the main stage(λth stage) illustrated in FIG. 12 is described, and then, a structureof the unit register circuits 38 in the dummy stages (k=1, 2, N−1, andN) is described mainly with reference to points different from thestructure in the main stage.

The unit register circuit 60 in the λth stage includes NMOS transistorsT1F, T1B, T2, T4 to T6, T6A, T6B, T6C, T9F, and T9B and capacitors C1and C3.

The unit register circuit 60 in the λth stage has an output terminalNOUT(λ) for outputting a pulse G(2λ−1) of its own stage. The unitregister circuit 60 in the λth stage further has, as terminals to whicha pulse in another stage is input, a forward direction set terminalNSF(λ), a reverse direction set terminal NSB(λ), a forward directionreset terminal NRF(λ), and a reverse direction reset terminal NRB(λ). Anoutput signal G(2λ−3) is input to the terminal NSF(λ) from the (λ−1)thstage, an output signal G(2λ+1) is input to the terminal NSB(λ) from the(λ+1)th stage, an output signal G(2λ+3) is input to the terminal NRF(λ)from the (λ+2)th stage, and an output signal G(2λ−5) is input to theterminal NRB(λ) from the (λ−2)th stage.

Further, output signals V(2λ−1), V(2λ+1), V(2λ+3), and V(2λ+5) are inputto the unit register circuit 60 in the λth stage from the clock signalgenerating portion 34. Further, to the unit register circuits 60, avoltage at the H level is supplied from a power supply VGH and a voltageat the L level is supplied from a power supply VGL.

A drain of the output transistor T5 is connected to a signal line of theclock signal V(2λ−1) and a source of the output transistor T5 isconnected to the output terminal NOUT(λ), and the conduction of thetransistor T5 is controlled according to the potential of a referencepoint N1 connected to a gate of the transistor T5. The capacitor C1 isconnected between the gate and the source of the transistor T5. Thetransistor T5 and the capacitor C1 function as an output circuit whichoutputs the output pulse G(2λ−1) of its own stage in synchronizationwith the clock pulse V(2λ−1) with the reference point N1 being at the Hlevel.

Drains of the transistors T6, T6A, T6B, and T6C are connected to theoutput terminal NOUT(λ) and sources of the transistors T6, T6A, T6B, andT6C are connected to the power supply VGL. A gate of the transistor T6is connected to the node N2. A clock signal V(2λ+1) is applied to a gateof the transistor T6A. The clock signal V(2λ+3) is applied to a gate ofthe transistor T6B. A clock signal V(2λ+5) is applied to a gate of thetransistor T6C. When the potential of the node N2, the clock signalV(2λ+1), the clock signal V(2λ+3), or the clock signal V(2λ+5) is at theH level, the output terminal NOUT(λ) is connected to the power supplyVGL.

The reference point N1 is connected to the terminals NSF(λ) and NSB(λ)via the diode-connected transistors T1F and T1B, respectively. Thetransistors T1F and T1B function as a set circuit which sets thereference point N1 to the H level when an output pulse of another stageis input to the terminal NSF(λ) or NSB(λ).

The transistors T2, T9F, and T9B which are connected between thereference point N1 and the power supply VGL so as to be in parallel toone another function as switch elements which providecontinuity/discontinuity between the reference point N1 and the powersupply VGL. A gate of the transistor T2 is connected to the node N2, agate of the transistor T9F is connected to the terminal NRF(λ), and agate of the transistor T9B is connected to the terminal NRB(λ). When thepotential of the node N2, the terminal NRF(λ), or the terminal NRB(λ) isat the H level, these transistors set the potential of the referencepoint N1 to the L level. In particular, the transistors T9F and T9Bfunction as a reset circuit which sets the reference point N1 to the Llevel when an output pulse of another stage is input to the terminalNRF(λ) or NRB(λ).

The transistor T4 is connected between the node N2 and the power supplyVGL. A gate of the transistor T4 is connected to the reference point N1.Further, the capacitor C3 is connected between the node N2 and an inputterminal of the clock signal V(2λ−1). The transistor T4 functions as aswitch element which provides continuity/discontinuity between the nodeN2 and the power supply VGL. During a period in which the potential ofthe reference point N1 is at the H level, the transistor T4 is in the onstate, and sets the potential of the node N2 to the L level. On theother hand, during a period in which the potential of the referencepoint N1 is at the L level, the transistor T4 is in the off state. Inthis state, when the clock signal V(2λ−1) is at the H level, thepotential of the node N2 is pulled up to the H level via the capacitorC3.

Next, the unit register circuits 60 in the dummy stages are described.For the same reason as in the case of the first embodiment, the unitregister circuits 60 in the μ-th stages (μ=1, 2), that is, in the topdummy stages, illustrated in FIG. 11 do not have the transistor T9B,and, correspondingly to this, do not have the terminal NRB. It is to benoted that the first and second stages have the terminals NSF, NSB, andNRF. Similarly to the case of the λth stage, a corresponding outputsignal in another stage is input to these terminals except the terminalNSF(1). Similarly to the case of the first embodiment, to the terminalNSF(1), a pulse of the forward direction trigger signal VSTF is inputfrom the trigger signal generating portion 36 at the start of a forwardshift.

The unit register circuit 60 in the μ-th stage (μ=(N−1), N), that is, ina bottom dummy stage illustrated in FIG. 13 does not have the transistorT9F for the same reason as in the case of the first embodiment, and,correspondingly to this, does not have the terminal NRF. It is to benoted that the (N−1)th and N-th stages have the terminals NSF, NSB, andNRB. Similarly to the case of the λth stage, a corresponding outputsignal in another stage is input to these terminals except the terminalNSB(N). Similarly to the case of the first embodiment, to the terminalNSB(N), a pulse of the reverse direction trigger signal VSTB is inputfrom the trigger signal generating portion 36 at the start of a backwardshift.

Further, as described in the first embodiment, it is preferred that thesize of the output transistors T5 in the dummy stages be made smallerthan that in the main stages.

In the above, the structure of the gate line driving circuits 14 isdescribed taking as an example the gate line driving circuit 14R fordriving the odd-numbered gate signal lines 20. The structure of the gateline driving circuit 14L for driving the even-numbered gate signal lines20 in this embodiment is similar to that of the gate line drivingcircuit 14R on the right side. In this regard, this embodiment issimilar to the first embodiment, and thus, the description thereof isomitted.

Next, operation of the bidirectional shift register 30 is described.Timing diagrams illustrating waveforms of various signals in the forwardshift operation and in the backward shift operation in this embodimentare the same as FIGS. 8 and 9, respectively, which are referred to inthe first embodiment.

The forward shift starts by, at the head of image signals for one frame,generation of a pulse of the forward direction trigger signal by thetrigger signal generating portion 36 (at the times t0 and t1 in FIG. 8).As described above, the clock signal generating portion 34 generates, inthe forward shift operation, pulses in sequence in the forwarddirection.

Here, first, forward shift operation of the unit register circuit 60 ina main stage (λth stage) in the gate line driving circuit 14R isdescribed.

Before the operation in the λth stage, the first to (λ−1)th stages areoperated in sequence to output pulses having a width of 2H with a phasedifference of 2H therebetween. When a pulse of the output signal G(2λ−3)in the (λ−1)th stage is input to the terminal NSF(λ) (time t2 in FIG.8), the reference point N1 is set to a potential (VGH-Vth (T1F))corresponding to the H level to turn on the transistor T5, and theinterterminal voltage of the capacitor C1 is set to the same potential.Here, the transistor T4 is turned on to set the node N2 to the L level.Therefore, the transistors T2 and T6 are in the off state.

At the time t3 after 2H from the time t2, a pulse of the output controlclock signal V(2λ−1) is input to the drain of the transistor T5. Thepulse of the clock signal V(2λ−1) pulls up the potential of the sourceof the transistor T5. Then, the potential of the reference point N1 isfurther pulled up by the bootstrap effect, and the pulse of the clocksignal V(2λ−1) becomes a pulse of the signal G(2λ−1) without loweringthe potential thereof to be output from the terminal NOUT(λ). The pulseof the signal G(2λ−1) is input to the terminal NSF in the (λ+1)th stage,and sets the reference point N1 in that stage to the H level.

When, at the time t4, the pulse of the output control clock signalV(2λ−1) falls, the pulse of the signal G(2λ−1) also falls. Further, atthis timing, a pulse of the clock signal V(2λ+1) turns on the transistorT6A, and thus, the output terminal NOUT(λ) is connected to the powersupply VGL and the output signal G(2λ−1) is at the L level. On the otherhand, the potential of the reference point N1 is held at the H level(subsequent set period).

At the time t4, the (λ+1)th stage outputs a pulse of the signal G(2λ+1)in synchronization with the pulse of the clock signal V(2λ+1). In thisway, each of the stages outputs a pulse of its own with a delay of 2Hfrom the output of a pulse in the preceding stage. The (λ+2)th stagewhich receives the output of the pulse in the (λ+1)th stage outputs apulse of the signal G(2λ+3) at a time t5 which is delayed by 2H from thetime t4.

In the λth stage, when the pulse of the signal G(2λ+3) is input to theterminal NRF at the time t5, the transistor T9F is turned on to resetthe reference point N1 to the L level. At the same time, a pulse of aclock signal V(2λ+3) turns on the transistor T6B to hold the outputsignal G(2λ−1) at the L level. It is to be noted that, in the subsequent2H period, the transistor T6C is turned on by a pulse of a clock signalV(2λ+5) to hold the output signal G(2λ−1) at the L level.

By the way, a pulse of the output control clock signal V(2λ−1) isapplied to the drain of the transistor T5 even outside the set period ofthe reference point N1 (output period of the λth stage), and the pulsehas a function to pull up the potential of the reference point N1 viathe gate-drain capacitance Cgd of the transistor T5. The potentialfluctuations at the reference point N1 are suppressed by turning on thetransistor T2. In a reset period of the reference point N1, thereference point N1 is basically at the L level and the transistor T4 isin the off state. In this state, as described above, the potential ofthe node N2 is pulled up to the H level according to a pulse of theoutput control clock signal V(2λ−1), and the transistors T2 and T6 areturned on. This fixes the reference point N1 to the L level in the resetperiod, and the output signal G(2λ−1) is held at the L level.

As described above, a main stage receives an output pulse from theprevious stage to cause the reference point N1 to be in the set state,and receives an output pulse from the stage next to the subsequent stageto cause the reference point N1 to be in the reset state. However, thedummy stage as the first stage does not have the previous stage.Therefore, as described above, the first stage has a structure in whicha pulse of the forward direction trigger signal VSTF is input to theterminal NSF. The first stage receives a pulse of the signal VSTF whichis generated at the time t0 to set the reference point N1 to the Hlevel. Operation in the first stage after this is similar to that in theλth stage described above. As described above, the (N−1) th and N-thstages which are dummy stages do not have the reset terminal NRF. Thereference point N1 in the (N−1) th and N-th stages is set to the H levelat the end of the forward shift operation for one frame in that stage,and then is reset to the L level when the transistor T2 is turned onbefore the start of the subsequent frame. More specifically, as aninconvenience which results from setting the preceding set period to belong, as described above, the potential of the reference point N1 in the(N−1) th and N-th stages is lowered by a leakage current of thetransistor T9B or T2. Here, the transistor T2 is, for example, designedto have a larger channel width than that of the transistor T4, and thus,the leakage current becomes larger accordingly. Further, a period inwhich the shift operation is suspended that occurs between framescorrespondingly to a vertical blanking period is relatively long, duringwhich the potential of the reference point N1 is lowered by a leakagecurrent.

It is to be noted that there may be employed another structure in whichthe transistor (denotes as Tr), which is turned on by a pulse of a clocksignal V(2μ+3) (μ=(N−1), N), is provided between the reference point N1and the power supply VGL in the (N−1)th and N-th stages which are dummystages, and the transistor Tr is turned on by a clock signal which isgenerated before the start of the shift operation for the subsequentframe to reset the reference point N1 in the dummy stages to the Llevel. In this structure, in the μ-th stage which is a dummy stage, anoutput pulse is generated in synchronization with a clock signalV(2μ−1). Taking into consideration that the reference point N1 cannot bereset to the L level at the timing of generating the output pulse andduring the preceding set period therebefore and the subsequent setperiod thereafter, respectively, the transistor Tr is controlled by theclock signal V(2μ+3) which generates a pulse at a timing which does notfall within such periods. Further, in the gate line driving circuit 14Rwhich is driven by four-phase clock signals, when the transistor T9Bwhich is controlled by the output signal G(2λ−5) is turned on, thetransistor Tr is also turned on, and thus, in the structure in which thetransistor Tr is provided, the transistor T9B in the (N−1)th and N-thstages which are dummy stages may be eliminated. Similarly, in the firstand second stages which are dummy stages (μ=1, 2), the transistor Tr mayalso be provided to reset the potential of the reference point N1 to theL level in the backward shift operation. In this embodiment, thetransistor Tr in the first and second stages which are dummy stages mayalso be controlled by the clock signal V(2μ+3), and the transistor T9Fmay also be eliminated.

The forward shift operation of the stages in the gate line drivingcircuit 14R is described above. The forward shift operation of thestages in the gate line driving circuit 14L is similar to that of thecorresponding stages in the gate line driving circuit 14R. However, thestages in the gate line driving circuit 14L operate at a timing 1Hbehind the corresponding stages in the gate line driving circuit 14R,respectively.

The backward shift starts by, at the head of image signals for oneframe, generation of a pulse of the reverse direction trigger signal bythe trigger signal generating portion 36 (at the times t0 and t1 in FIG.9). As described above, the clock signal generating portion 34generates, in the backward shift operation, pulses in sequence in thereverse direction.

Similarly to the case of the unit register circuits 38 in the firstembodiment, in the unit register circuits 60 in each of the main stagesof the shift register portion 32, the set terminals and the resetterminals are in a symmetrical relationship in structure with respect tothe forward shift and the backward shift. Further, similarly to thefirst embodiment, the top dummy stages and the bottom dummy stages arein a symmetrical relationship in structure with respect to shifts inopposite directions. Therefore, if the control circuit 18 switches thetrigger signals and switches the order of generation of the clockpulses, the shift register portion 32 performs the backward shiftoperation similarly to the forward shift operation.

For example, in the N-th stage in the gate line driving circuit 14R, apulse of the reverse direction trigger signal VSTB is input to theterminal NSB at the time t1 and the reference point N1 is set to the Hlevel. After that, in synchronization with a pulse of a clock signalV(2N−1) which is generated first, a pulse is generated in an outputsignal G(2N−1). In this way, pulses are output from the stages insequence in the opposite order to that in the forward shift operation.

The backward shift operation is described in the above taking as anexample the gate line driving circuit 14R. The backward shift operationof each of the stages in the gate line driving circuit 14L is similar tothat of the corresponding stage in the gate line driving circuit 14R.However, each of the stages in the gate line driving circuit 14Lperforms operation with an advance of 1H from the corresponding stage inthe gate line driving circuit 14R.

It is to be noted that various kinds of variations in structuredescribed in the first embodiment may also be adopted in thebidirectional shift register according to this embodiment.

Further, the transistor Tr described as a transistor in the dummy stagesmay also be adopted in the main stages to eliminate at least one of thetransistors T9F and T9B in the main stages. In particular, in theabove-mentioned gate line driving circuits 14 which are driven byfour-phase clock signals (n=4 and βf=βb=2), by providing one transistorTr which is controlled by the clock signal V(2λ+3), both of thetransistors T9F and T9B may be eliminated. Such a structure in which onetransistor Tr is provided to eliminate both of the transistors T9F andT9B is possible generally when βf+βb=n.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

1. A bidirectional shift register, comprising: a shift register portionincluding unit register circuits each of which has an output terminalfor outputting a pulse and which are cascaded in m stages, where m is aninteger which is equal to or larger than 6, for outputting the pulsefrom a column of the output terminals in a shift order which is one of aforward direction and a reverse direction; a clock signal generatingportion for supplying n-phase clock pulses, where n is an integer whichis equal to or larger than 3, to the stages of the shift registerportion in sequence in the forward direction in forward shift operationof the shift register portion and in the reverse direction in backwardshift operation; and a trigger signal generating portion for generatinga forward direction trigger signal at a start of forward shift andgenerating a reverse direction trigger signal at a start of backwardshift, the cascaded unit register circuits including main stages thatare unit register circuits in which a load to be driven is connected tothe output terminal and dummy stages that are unit register circuits inwhich a load to be driven is not connected to the output terminal, themain stages forming a successive column and the dummy stages beingconnected at a top before the column of the main stages and at a bottomafter the column of the main stages in the cascade, when αf, βb, βf, andβb are natural numbers which satisfy both αf<βb<n and αb<βf<n, the unitregister circuit in a k-th stage, where k is an integer which satisfies1≦k≦m, including: an output circuit for outputting an output pulse P_(k)in synchronization with the n-phase clock pulse which is input to theunit register circuit with a reference point of the unit registercircuit being at a first potential; both a forward direction setterminal to which an output pulse P_(k−αf) is input as a set signal anda reverse direction set terminal to which an output pulse P_(k−βb) isinput as a set signal; at least one of a forward direction resetterminal to which an output pulse P_(k+βf) is input as a reset signaland a reverse direction reset terminal to which an output pulse P_(k−βb)is input as a reset signal; a set circuit for setting a potential of thereference point to the first potential when the set signal is input; anda reset circuit for setting the potential of the reference point to asecond potential when the reset signal is input, each of the main stagesincluding both the forward direction reset terminal and the reversedirection reset terminal, a number of the top dummy stages providedbeing βb, each of the top dummy stages including only the forwarddirection reset terminal as the reset terminal, a number of the bottomdummy stages provided being βf, each of the bottom dummy stagesincluding only the reverse direction reset terminal as the resetterminal, and the forward direction trigger signal instead of the outputpulse P_(k−αf) being input as the set signal to the forward directionset terminal in top of stages of the dummy stages, and the reversedirection trigger signal instead of the output pulse P_(k+βb) beinginput as the set signal to the reverse direction set terminal in bottomαb stages of the dummy stages.
 2. The bidirectional shift registeraccording to claim 1, wherein: the reset circuit includes a firsttransistor provided correspondingly to the forward direction resetterminal and a second transistor provided correspondingly to the reversedirection reset terminal, each of the first transistor and the secondtransistor being provided between the reference point and a power supplyof the second potential and, when the reset signal is applied to a gateterminal of the each of the first transistor and the second transistor,being turned on to connect the power supply to the reference point; andthe dummy stages includes only one of the first transistor and thesecond transistor.
 3. The bidirectional shift register according toclaim 2, wherein the first transistor and the second transistor comprisea-Si transistors.
 4. The bidirectional shift register according to claim1, wherein: the output circuit in each of the stages includes an outputtransistor for generating, according to output of the pulse from theoutput terminal in the stage, a driving current at the output terminal;the output transistor in the main stages has a driving abilitycorresponding to the load to be driven; and a driving ability of theoutput transistor in the dummy stages is set to be lower than thedriving ability of the output transistor in the main stages.
 5. Thebidirectional shift register according to claim 1, wherein both of andab are
 1. 6. An image display device, comprising: a plurality of pixelcircuits arranged in matrix correspondingly to a plurality of scanninglines; a plurality of gate signal lines provided for the plurality ofscanning lines, respectively, for supplying a gate signal forcontrolling writing of image data to the plurality of pixel circuits;and a gate signal line driving circuit including the bidirectional shiftregister according to claim 1 in which the plurality of gate signallines are connected to the output terminals in the main stages,respectively, for generating the gate signal based on the pulse which isoutput from the main stages of the shift register portion.